Method of heat-treating gray cast iron



Patented Sept. 11, 1951 METHOD OF HEAT-TREATING GRAY CAST IRON Frederick-G. .Sefing, Cranford, N. 'J-, .assignorito The International NickelCompany, ,Inc., New York, N. Y a corporation ,of Delaware No Drawing,Application August '10, 1949,

SerialNo. 10.9;613

The present invention relates .to the production of special hardenedgray cast iron compositions and castings made thereof characterized byhigh hardness and by high resistance to the effects of corrosion,abrasion and wear.

It has now been discovered that a special al- 10y gray cast ironcomposition can be softened to a Satisfactory hardness level formachining and is vthereafter, in cooperation with a special thermalvtreatment,hardened to a high hardness level, e. g., about ,450.Brinellor more.

It is an obiectof thepresent invention to provide alloy gray ,cast ironcastings which can be machined and which thereafter can be hardened to ahigh hardness level. i W

Another object of the invention is ,to provide a hardenedgray cast ironresistant to the corrosive effects of salty, acidic or basicenvironments. r

The invention also contemplates providing a hardened gray cast ironresistant to the deleterious action of abrasives and resistant tometal-to-metal wear.

It is a further object of the invention to provide a special thermalhardening treatment cooperating with a special alloy gray cast ironcomposition to produce a high hardness level therein.

Broadly stated, the invention comprises a special hardened nickel-copperalloy gray cast iron characterized by a high hardness and a specialprocess for the production of said hardened alloy'gray cast iron. Thespecial hardened a1- loy gray cast iron provided by the inventioncontains about 2.25% to about 3.25% total carbon,

about 7% to about 10.5% nickel, about 3% to about 4.7% coppenabout 1% toabout 4% chromium, about 1.5% to about 3% silicon, about 0.3% to about1% manganese, and the balance essentially iron, said balance being atleast about 73% iron. I

The special alloycast iron composition provided by the invention ishardened to a high hardness level by a special treatment which comprisessubjecting castings made of said cast iron to sub-Zero temperatures, e.g., temperatures of about minus 70 F. to about minus 320 F. or lower,for relatively short periods of time, e. g., for periods of timesufficient to thoroughly cool saidcastings to the temperature of thetreating medium.

In carrying the invention into practice, it is preferred to employ alloycast iron compositions containingabout 2.5% to about 3% total carh about75% i a ou 9% b i 3.

2"ClaimS; (Cl. 148-2137) to about 4% copper, about 12% .to about 25%chromium, about .1;% to about 2.25% silicon, up to about 0.8% manganese,and the balance essentially iron. .T-heaforesaid .preferred hardenedalloygraycast iron. compositions are characterized by improvedvhardening in the subzerotreatment and .by improved corrosion,resistance.

Castings ,containingiless than about 12% copper plus nickel and/or morethan about 2.5% chromiumhave been .found frequently to be undesirablyhard and/or, to have a non-uniform hardness in the as-cast condition,with the recult that such castingsare machinable only with difilculty.It is desirable therefore, before machining such ,castings, to heat saidcastings to a temperature of about to; about .1800" F., e. g., 1700813and to thereafter rapidly cool said castings, e. g., to oil quench saidcastings. The aforesaid rapidcoolin gpr quenching iscarried out ata rate.ofnot less than about 325 F. per second (at 1300 F.) and thetemperature of the castings is reduced belowrabout 900 F. by means ofthe rapid cooling treatment. The quenching treatment is desirable toovercome the effect of possible production variables and to insure thatthe cast compositions provided by the processembodying the inventionwill have a uniform mac'hinable hardness level within a range of about160 to about 250 -Brinell, e. g., a hardness levelof about 225 Brinell.In those-castings having asatisfactory-machinable hardness in the --as-cast condition, the-aforesaid quenching treatment is not essential toinsure hardening :toa satisfactory hardness llevel upon the subsequentsub-zero treatments The special process contemplated by the inventionthus provides alloy gray cast iron castings which may bethermallysoftened to a satisfactory machinable hardness level and whichmay be hardened after any required machining to a high hard-ness'level.0.004 inches per inch usually occurs duringthe thermal hardeningtreatment. As indicated hereinbefore, the special compositionscontemplated by the inventioncooperate with the special hardeningtreatment-providedbythe invention to. produce graycast iron castingshaving high hardness, e. g., castings having hardnesses of about 450 toabout.575 iBrinell are produced according to the invention. 'In general,it can be said that when the higher .hardnesses are required, e.g.,,hardnesses of about 450 Brinell-or more, thelower treatingtemperatures should do employed in the hardening operation,

"An expansion of about The amounts of alloying elements in the cast ironcompositions contemplated by the invention set forth hereinbefore arecritical and the novel results provided by the invention are notachieved unless the aforesaid amounts of alloying elements are employed.Thus, when the carbon content is below about 2.25%, trouble isencountered by way of poor castability, poor machinability and poormetal-to-metal wear resistance. On the other hand, at carbon contentsabove about 3.25%, the high hardness values contemplated by theinvention are not achieved. The total content of nickel plus copper inthe special alloy castiron provided by the invention is maintainedwithin the range of about to about because when the total content ofthese two elements is substantially below about 10%, quenched castingsare too hard to be machinable, and when the total content of these twoelements is substantially above about 15%, the castings are nothardenable to a high level, e. g., a level of 450 Brinell or above. Thechromium content in combination with the total content of nickel pluscopper is also a critical factor as it has been found that with thetotal nickel plus copper contents set forth hereinbefore, castingscontaining less than about 1% chromium are not hardenable to a highhardness value and, on the other hand, castings containing more thanabout 4% chromium are not practically machinable, even after the rapidcooling treatment described hereinbefore.

. as when the silicon content exceeds about 3% it is diflicult toachieve the machinable hardnesses of less than about 250 Brinell in theascast and/or quenched conditions. The special alloy gray cast ironcastings provided by the invention contain at least about 1% graphite upto about 2.6% graphite, e. g., the preferred compositions set forthhereinbefore will contain about 1.25% to about 2% graphite. For bestresponse to the sub-zero hardening treatment, it is preferred to keepthe manganese content low, e. g., below about 0.5%.

For the purpose of giving those skilled in the art a better appreciationof the advantages of the invention, the following illustrative examplesare given:

Example 1 A gray cast iron casting containing about 2.6% total carbon,about 1.7% silicon, about 7.6% nickel, about 3.4% copper and about 2%chromium was produced. In the as-cast condition, this casting had ahardness of about 255 Brinell. The casting was heated to about 1700 F.and was quenched in oil. The hardness of the quenched casting was about200 Brinell. The casting was then hardened by immersion in a bath havinga temperature of about minus 70 F. After the hardening treatment, thecasting had a hardness of 429 Brinell. After hardening at minus 320 F.,another casing of the same composition and similarily oil-quencheddeveloped a hardness of about 514 Brinell.

4 Example 2 A gray cast iron casting containing about 2.6% total carbon,about 1.7% silicon, about 9.3% nickel, about 4.1% copper and about 2%chromium was produced. In the as-cast condition, this casting had ahardness of about Brinell. The casting was hardened by immersion in abath of liquid nitrogen at a temperature of about minus 320 F. After thehardening treatment, the casting had a hardness of about 485 Brinell.

In addition to the alloying elements set forth hereinbefore, the alloygray cast iron provided by the invention may also contain usual small.amounts of incidental impurities and other elements found in gray castiron and which do not materially affect the properties of the castingsprovided by the invention. Thus, the castings may contain up to about0.14% sulfur, up to about 0.5% phosphorus, etc.

Castings contemplated by the invention may be produced by the usualmethods and in the usual equipment employed in the production of highquality gray iron castings. Thus, the are or induction electricfurnaces, etc., have been employed successfully in the production ofcastings contemplated by the invention.

The special hardened alloy gray cast iron compositions provided by theinvention having the high hardness described hereinbefore arecharacterized by a high combination of corrosion resistance, abrasionresistance, metal-to-metal Wear resistance and heat resistance. Thus,the special cast iron has been found to be particularly resistant to thecorrosive effect of salt water and to have a highly useful resistance tothe corrosive effects of both acids and alkalies.

The high combination of corrosion resistance, abrasion resistance andmetal-to-metal wear re-.

sistance which characterize the hardened alloy geously made from the newcast product pro-.

vided by the invention. In addition, parts for equipment used inhandling abrasive materials at elevated temperature are also made withgreat advantage from the new product.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to Without departing from the spirit andscope of the invention as those skilled in the art will understand. Suchmodifications and variations are considered to be within the purview andscope of the invention and of the appended claims.

I claim:

1. The improved process for producing a hard, wear-resistant, cast ironcasting which comprises subjecting an alloy gray cast iron castingcontaining about 2.5% to about 3% carbon, about 7.5% to about 9% nickel,about 3.5% to about 4% copper, about 2% to about 2.5% chromium, about1.75% to 2.25% silicon, and the balance essentially iron to a thermaltreatment comprising heating said casting to a temperature of about 1600F. to about 1800 F., rapidly cooling said heated casting to atemperature below about 900 F. to soften said casting to a machinablehardness level of about 180 to about 250 Brinell, and there.

corrosion-resistant alloy gray after subjecting said casting to ahardening treatment at a temperature below about minus 70 F. to hardensaid casting to a, hardness of at least about 450 Brinell.

2. In the process for producing a hard, wearresistant,corrosion-resistant alloy gray cast iron casting, the improvement whichcomprises thermally treating a gray cast iron casting containing about2.25% to 3.25% carbon, about 7% to about 10.5% nickel, about 3% to about4.7% copper, the sum of said nickel and said copper contents being about10% to about 15%, about 1% to about 4% chromium, about 1.5% to about 8%silicon, up to about 1% manganese, and the balance essentially iron, ata temperature of about 1600 F. to about 1800" F., rapidly cooling saidthermally-treated casting below about 900 F. to soften said casting to alow machinable REFERENCES CITED The following references are of recordin the file of this patent:

UNITED STATES PATENTS Name Date Sefing Nov. 21, 1950 OTHER REFERENCESMachinery, January 8, 1948, page 43. "Cold-Treating Practice withDeepfreeze, page 13, published by Motor Products Corp., 1944.

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1. THE IMPROVED PROCESS FOR PRODUCING A HARD, WEAR-RESISTANT,CORROSION-RESISTANT ALLOY GRAY CAST IRON CASTING WHICH COMPRISESSUBJECTING AN ALLOY GRAY CAST IRON CASTING CONTAINING ABOUT 2.5% TOABOUT 3% CARBON, ABOUT 7.5% TO ABOUT 9% NICKEL ABOUT 3.5% TO ABOUT 4%COPPER, ABOUT 2% TO ABOUT 2.5% CHROMIUM, ABOUT 1.75% TO 2.25% SILICON,AND THE BALANCE ESSENTIALLY IRON TO A THERMAL TREATMENT COMPRISINGHEATING SAID CASTING TO A TEMPERATURE OF ABOUT 1600* F. TO ABOUT 1800*F., RAPIDLY COOLING SAID HEATED CASTING TO A TEMPERATURE BELOW ABOUT900* F. TO SOFTEN SAID CASTING TO A MACHINABLE HARDNESS LEVEL OF ABOUT180 TO ABOUT 250 BRINELL, AND THEREAFTER SUBJECTING SAID CASTING TO AHARDENING TREATMENT AT A TEMPERATURE BELOW ABOUT MINUS 70* F. TO HARDENSAID CASTING TO A HARDNESS OF AT LEAST ABOUT 450 BRINELL.